This invention relates generally to rock bits used in earth drilling for mining, the drilling of oil, gas and geothermal wells, and construction drilling; more specifically, it provides self-sharpening drag bits to be used for such drilling.
Rock bits are the most crucial components of earth drilling systems, as they do the actual cutting. Their performance determines the length of time it takes to drill to a given depth, thus, the efficiency of the drilling operation is largely dependent on the efficiency of such bits. Conventional bits are generally designed with three cone-shaped wheels, called "cones", with hardened steel teeth or carbide teeth for cutting the rock. For drilling very hard formations, a special bit with a diamond-studded face often replaces the tricone roller bit. The cones serve as cutters and utilize carbide or hardened steel teeth as the cutting elements. As the bit rotates, the cones roll around the bottom of the hole, each cutting element intermittently penetrating into the rock, crushing and chipping it. The cones are designed so that the teeth intermesh to facilitate cleaning. Drilling fluid pumped through cone nozzles carries away the cuttings.
Inasmuch as such drilling occurs in very harsh environments and under heavy loads, wear and tear effects of the rock formation on the bit are tremendous. The bit, therefore, must have an extremely durable mechanism and be made of materials that can withstand erosion and wear in places where the bit contacts the rock, such as the outside surface of the cones. In this regard, excessive erosion of the cone surface may cause cutting elements to fall off.
Cones rotate around a rugged set of bearings which, in most applications, must be protected from rock cuttings by a seal and lubricated for better performance. Excessive wear of either the seal or the bearings results in loss of the sealing function, and can quickly lead to premature bit failure. In carbide tipped bits, the cutting elements consist of tungsten carbide inserts and are press fitted into precisely machined holes drilled around the cone. The dimensions of the holes and the inserts must be precisely matched. If the fit is too tight, the insert or the cone may be damaged; if it is too loose, the inserts will fall off during drilling.
Similar requirements exist for milled tooth bits, except that in this case the cutting elements are teeth-machined from the cone body. Parts of the teeth are hardfaced, by welding a harder alloy layer to the surface to impart resistance to wear. This welded layer is usually non-uniform in thickness and composition. In most cases, portions of the cone surface are hardened by carburizing, which may last typically 10-20 hours at high temperatures affecting the properties of the whole part. Bearings are inlayed by welding or the bearing races are either carburized or boronized, which again require long thermal treatments. As the cutting elements wear, cutting efficiency decreases until the rate of penetration is too slow to economically justify further drilling. Then, the bit is pulled out and replaced. Raising and lowering of the drill string, called tripping, is a costly operation which may be reduced by extending the drill bit life and improving the efficiency of drilling.
The cutting elements on rollerbits begin to lose their sharpness soon after drilling begins. This shortcoming has been addressed by utilizing polycrystalline diamond drag bits or "PDC" drag bits for short. Major drawbacks of PDC drag bits include their inability to drill hard formations due to chipping and breakage of the diamond compacts, the high cost of the bit, and the excessive body erosion which may lead to cutter loss.
Deficiencies of existing rock bits include:
(1) Performance in roller bits depends on perfect working of several interdependent components. These are:
a bearings system PA1 a cutting structure PA1 a sealing mechanism PA1 a lubrication system PA1 (a) Inserts have to be press fitted, a precise yet still imperfect practice for carbide tipped roller bits; PA1 (b) Long thermal treatments, such as carburizing, can produce metallurgical side effects and distortion; PA1 (c) Hardfacing of milled steel teeth rarely produces a uniform deposit, both in dimensional or chemical points of view; PA1 (d) Bearing inlays too, are chemically non-uniform, thus, inherently weak.
Failure of any one of these affects the others, and leads to premature bit failure.
(2) Cutting efficiency is compromised due to several state-of-the-art necessities, listed as follows:
(3) Undesirably excessive machining and dimensional inspection require high labor use, and therefore result in high manufacturing cost.
(4) In all existing bit designs, the cutting elements represent only a small proportion of the total bit structure. When cutting elements wear out, the entireties of the bits become useless. Furthermore, in roller bits, especially, cutting elements dull quickly, resulting in a steady drop-off in drilling efficiency.
(5) In roller bits, the cutting structure has to fit into a limited space. This space is shared by the cones and the journal pins. Design changes based on increase in volume of both of these two components are impossible. Any volume change of one has to be made at the expense of the other. This limitation does not exist for drag bits.